The present invention relates generally to a luminance control circuit for a television receiver, and is directed more particularly to a luminance control circuit for a television receiver in which the limited dynamic range of the phosphor on a cathode ray tube is optimally utilized to reproduce a broadcast video signal.
The phosphor of a cathode ray tube has a limited dynamic range for the production of luminance in response to video signals applied thereto. At a particular minimum video signal level called the cutoff level, negligible luminance is produced by the phosphor. Further reduction in video signal level makes negligible perceptible change on the cathode ray tube. At a particular maximum video signal level, called the saturation level, the phosphor is providing all of the luminance of which it is capable. Further increase in video signal level again makes negligible perceptible change on the cathode ray tube. The dynamic range of a cathode ray tube is defined as the range between the cutoff and saturation levels.
For optimum reproduction of a broadcast video signal, the maximum luminance, called the white peak level, should coincide with the saturation level of the tube, and the minimum luminance, or black level, should coincide with the cutoff level of the tube. These conditions have been difficult to achieve since broadcast video signals do not always contain white peak and black levels which are consistent with the capabilities of certain cathode ray tubes.
A further problem occurs when the average beam current in a cathode ray tube exceeds a predetermined value. The high voltage circuit and/or the phosphor may be damaged by such overload.
In the prior art, limiter circuits have been employed to limit the peak beam current to a value which retains it within the dynamic range of the cathode ray tube. When a single limiter circuit is used on the luminance signal prior to using it to control a color matrix, high intensity color difference signals applied to the matrix circuit may produce beam currents which are determined more by such color difference signals than by the level of the luminance signal. Thus the video signal may not be limited to below the saturation level. If a separate limiting circuit is used on each of the three color signals fed to a cathode ray tube, when the level of one of the colors exceeds the limiting level and the others do not, the high-level color is proportionately deleted from the color picture reproduced by the cathode ray tube. This changes the color of the reproduced image to induce color in a white signal or to distort the color in a colored signal.